My guess on the constant throttle blipping, other than the "LOOK AT ME" aspect of it, is that after fitting the bike with straight pipes it actually leans out the fuel mix. If the bike owner didn't have the carbs/fuel injection adjusted to suit, the bike probably idles for crap without the constant shot of more gas.

No, you've got it all wrong.

In the past I always thought that it was just such a piece of shit bike that it wouldn't idle. It seemed obvious to me that it was poorly maintained, else how did they lose the baffle?

This still may be the case for many of these bikes, but I've come up with an alternative theory.

Shortly after installing loud pipes the rider is soon suffering from partial or total hearing loss and is unable to hear their bike idling. Naturally, they think it may have stalled. They blip the throttle just to make sure it is still running.

That, or they are just trying to compensate for some other anatomical deficiency.

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